Thermal effects are ubiquitous, and have great effects on many systems. Thermal expansion, and in addition differences in coefficients of thermal expansion (CTE”s) between components, can lead to errors and other problems. An example is a spindle in a wafer inspection system. Due to the high wafer rotation speed and therefore high spindle power, there is a large resulting heat rise during wafer inspection. Any change in temperature affects the position of a component such as the spindle, due in part to the mounting materials' CTE. This change in position can degrade inspection accuracy and sensitivity. For example, for an aluminum system, given the size of the spindle and the CTE of aluminum, a 2 degrees C. temperature shift can result in XY accuracy out of spec. Similar thermal effects can have negative impact on many other mechanical systems such as machine tools. Attempts have been made to minimize thermal displacement by using materials with a very low CTE such as Invar, a nickel steel alloy notable for its uniquely low coefficient of thermal expansion. However, use of such materials as Invar creates new problems. If a portion of the system is Invar, with a CTE approximately zero, and the rest of the system is aluminum with a CTE of 24 um/m/° C., this creates a sensitivity to environmental temperature change. And Invar is prohibitively expensive to use for serious structural work. Keeping everything aluminum eliminates this problem.
Some efforts have been made to correct for thermally-induced errors on machine tools via software. No such corrections have been implemented for wafer inspection systems such as Surfscan from KLA-Tencor. As customer requirements have driven wafer inspection throughput requirements to higher levels, the necessary increase in spindle power has resulted in an increased heat load, to the point that correction of thermal effects has become very important for maintaining defect location accuracy.